The present invention relates to an electric power steering system for a motor vehicle, and more particularly to a system for controlling an electric motor in accordance with the turning of a steering wheel.
U.S. Pat. No. 4,664,211 discloses an electric power steering system. The system is provided for producing an assist signal dependent on the torsion torque signal from a torsion torque sensor and on the vehicle speed, an auxiliary assist signal dependent on the differentiation of the torsion torque signal, a return torque signal depending on an output signal of a steering angle sensor, for returning the steering wheel to a neutral position, and a damping signal depending on the differentiation of the steering angle signal. These signals are added for controlling the motor.
The auxiliary assist signal and the damping signal are provided for compensating the inertia of the motor. However, since each signal is dependent on the differentiation of the output of the sensor, the system produces the signal in response also to a small output of the sensor which is caused by noises during driving. Such a signal is generated even if the vehicle travels straight, so that vain current flows in the system, consisting electric power.